Protection in an ant–plant mutualism: an adaptation or a sensory trap?

Many traits of ant plants and their ant symbionts are thought to be coevolved, but there is little evidence for adaptation in these symbioses. We investigated the ant trait of worker attraction to, and consequent patrolling of, new plant shoots, and we tested two hypotheses to explain the maintenance of this trait. (1) New shoots chemically mimic ant brood or alarm pheromones (a ‘sensory trap’) and thereby elicit worker patrolling of vulnerable plant parts. (2) Worker attraction to new shoots is the result of selection on the ant to direct patrolling to the plant parts that maximize the capture of plant-provided rewards. As our model system, we used the ant plant Cordia nodosa and its protecting ant symbiont Allomerus octoarticulatus var. demerarae. Gas chromatography analyses suggested that compounds were shared between new leaves and Allomerus brood, and Allomerus workers were attracted to brood extracts of nonself colonies, findings that are consistent with the sensory trap hypothesis. However, patrolling Allomerus workers were attracted only to new leaves, whereas brood-tending workers collected from inside plant domatia (‘nurses’) were attracted to Allomerus brood rather than to new leaves. Only patrollers were attracted to new leaves significantly more than to mature leaves, and nurse workers were larger than patroller workers, which suggests that the behavioural differences reflect caste differentiation. Therefore, we reject the sensory trap hypothesis. Our results are consistent with the idea that worker attraction to new shoots is the result of selection.